This invention relates to novel urethane-modified polyisocyanate mixtures of the diphenylmethane series; to a process for their preparation by reaction of polyisocyanates or polyisocyanate mixtures of the diphenylmethane series with certain monohydric and/or polyhydric alcohols and, optionally, subsequent mixing of the modification products with polyisocyanates or polyisocyanate mixtures of the diphenylmethane series free from urethane groups; and to the use of the polyisocyanate mixtures in the preparation of polyurethane foams, particularly rigid polyurethane foams.
Polyisocyanates or polyisocyanate mixtures of the diphenylmethane series have been used for many years as polyisocyanate component in the production of polyurethane foams, particularly rigid polyurethane foams. The term "polyisocyanate of the diphenylmethane series" is the generic term for all diisocyanates and polyisocyanates that are formed in the phosgenation of aniline/formaldehyde condensates and which are present as a mixture in the phosgenation products. The term "polyisocyanate mixture of the diphenylmethane series" encompasses mixtures of "polyisocyanates of the diphenylmethane series", that is, the above-mentioned phosgenation products of aniline/formaldehyde condensates and mixtures formed by mixing individual "polyisocyanates of the diphenylmethane series" and/or various mixtures thereof, as well as mixtures of "polyisocyanates of the diphenylmethane series" of the type formed as distillate or distillation residue in the partial distillation of phosgenation products of aniline/formaldehyde condensates.
Polyisocyanate mixtures of the diphenylmethane series which have an average NCO functionality of approximately 2 to 3.1 and a viscosity at 25.degree. C. in the range from about 50 to 2,000 mPa.cndot.s (preferably in the range from 100 to 700 mPa.cndot.s) are preferably used for the production of rigid polyurethane foams. Rigid polyurethane foams, which are used for heat insulation, have previously been prepared from such polyisocyanate mixtures of the diphenylmethane series, suitable polyhydroxyl compounds, blowing agents, and other auxiliaries and additives. Hydrogenfree "chlorofluorocarbons", such as monochlorotrifluoromethane for dichlorodifluoromethane, for example, have been used as the blowing agents because of their low thermal conductivity and their high compatibility with the starting materials.
The thermal conductivity, and hence the heat insulation of polyurethane foams, depends not only on the thermal conductivity of the blowing gas filling the closed cells but also on the diameter of the polyurethane cells. The smaller the diameter and the more uniform the cell structure, the lower will be the thermal conductivity of the foam. A disadvantage of using "alternative" blowing agents, such as low-boiling hydrocarbons (for example, propane, butane or pentane) and/or hydrogen-containing chlorofluorocarbons (such as monochlorodifluoromethane) and/or water, is that, in typical formulations, these blowing agents can lead to comparatively large cell diameters and irregular cell structures. Moreover, the resultant foams show a tendency to form non-uniform surface skins.
It has now surprisingly been found that certain urethane-modified polyisocyanate mixtures based on polyisocyanate mixtures of the diphenylmethane series and certain mono- and/or polyhydric alcohols are excellent starting materials for the preparation of polyurethane foams, particularly rigid polyurethane foams and allow the preparation of fine-cell foams with closed cells having none of the disadvantages discussed above, even when using "alternative" blowing agents of the type mentioned, particularly hydrogen-containing chlorofluorocarbons and/or low-boiling aliphatic hydrocarbons and/or water.